The invention relates to a method for printing with an inkjet ink on on-porous surfaces of a substrate.
Until now, inket inks were printed directly onto materials or substrates having a non-porous surface. The inkjet inks may contain components which produce or improve adhesion to these surfaces. JP-A-078472 describes a transparent receiver medium which has an imprintable transparent coating. The transparent receiver medium is characterized by light permeability in the visible range of more than 80%. A method for producing water-proof markings on surfaces of plastic materials is disclosed in WO 99/18163. An ink is hereby utilized which contains a not easily volatilized solvent, which is absorbed, to a large extent, by the plastic material. In DE-A-3721651, a method is proposed for printing on plate-shaped information carriers with label information. The method is characterized in essence in that the label information is furnished by means of a data processing system. EP-A-413442 describes an ink for the marking of plastic material, whose composition comprises an organic solvent. The employed solvent leads to separation or swelling of the plastic material to be printed.
In the known proposals, adhesion continues to be unsatisfactory. In addition, so-called massive outflow leads to low resolution, with long drying times. lnkjet inks with brief drying times lead to clogging of the print head jets. Colored or black surfaces cannot be printed according to the traditional three-color-print, since white as a mixed color is missing on the substrate. At the present time there is no satisfactory solution for a white inkjet ink. Currently, no useable technical proposal exists in order to remedy these problems. The invention was therefore based on the object of improving the initially mentioned method in such fashion that it is possible to imprint non-porous surfaces of any type of substrate, specifically of plastic, metal, glass and ceramic and similar, with an inkjet ink, while achieving, for the ink, brief drying times, good adhesion and resistance to water. In addition, high resolution is to be sought. The disadvantageous bleeding of the ink on the non-porous surface of the substrate shall be excluded to a large extent.
The present invention solves this object in that the non-porous surface, coated with a varnish, is printed with an inkjet ink containing a varnish-etching component. The nature of the invention, consequently, consists in providing the non-porous surface of the substrate with a varnish layer. The type of varnish does not play a decisive role. Instead, one only needs to pay attention to the fact that between the respectively employed varnish and the inkjet ink employed for printing there is coordination to the extent that in the inkjet ink there is contained a component which etches or swells the varnish on the non-porous surface of the substrate. Generally, this is the solvent of the inkjet ink proper. Additionally, it is also conceivable that an additional component is contained in the inkjet ink which causes the etching or swelling of the varnish coating. This may involve, for example, alcohols, glycol-esters, esters, ketones or chlorated hydro-carbons.
In the individual case it is necessary to add to the varnish pigments and/or coloring substances, in order to achieve the desired color shade. In such instance, one also speaks of varnish paints (compare DIN 55 945, December 1988). Basically, varnishes are liquid or powdery solid substances, which are applied in thin layer on articles or which form a solid film adhering to the objects by chemical reaction and/or physical modification, said film having a decorative and/or protective function. The main components of the varnish are binders, solvents, pigments, fillers and varnish auxiliaries. In case of non-covering clear varnish, whose color depends upon that of the binder, the pigment is omitted, in case of solid varnish in powder form, the solvent is omitted. According to DIN 55 945 (December 1988) varnishes are coating substances based on organic binders. Depending upon the type of organic binder, varnishes may contain organic solvents, and/or water or contain neither. According to the meaning of the invention, included in varnishes are also xe2x80x9ccolored varnishesxe2x80x9d which originate from so-called lacquerification. Varnish-hardening plays hereby a role. The hardening into stable surfaces of varnishes applied in liquid state is called varnish-hardening. Varnish-hardening can take place either by air drying (physical drying), by oxidative cross-linkage (atmospheric moisture hardening) orxe2x80x94as in the case of the hardening of plasticxe2x80x94by polymerization (unsaturated polyesters/polystyrols), poly-addition (polyisocyanate/polyhydroxy-compounds or poly-epoxide/polyamine) or by poly-condensation (alkyd or acrylate resins with amine resins). As binders in varnishes are mostly used refined natural products, for example made of colophonium and oil or cellulose nitrate (nitro-varnishes) and totally synthetically constructed resins, (artificial resins). The classic natural resins, such as cupale, amber and shellac have currently only little technical application significance.
Employed as synthetic resins are specifically phenol resins, amine resins (for example benzo-guanamine-, urea-, melamine-resins), alkyd-resins, polyvinylacetate, epoxide resins, polyurethane-resins, polyester-resins, with colophonium-modified phenol-resins, chlorinated rubber, chlorinated polypropylene, cyclo-rubber, ketone resins, acrylate resins. Solvents or thinners in applicable resins according to the invention are liquids in which binders are dissolved in order to bring varnishes into a better applicable form. During drying the of varnish coating, these solvents almost fully evaporate, with the varnish remaining behind as film, either cross-linked or not-cross-linked, with glossy, matte or satiny appearance. Being considered as solvents are, in particular, ether-alcohols, aromatics, chlorated hydrocarbons, esters, hydro-aromatics, ketones, terpene hydrocarbons, andwater among others. Generally, several of these solvents are combined.
As already states, these varnishes can be pigmented, i.e. they can be mixed with the traditional solvents, insolvable organic or inorganic pigments, for example titanium dioxide, ferrous oxide red, ferrous oxide black, chromium oxide green, molybdate, cadmium pigments, carbon blacks, azo-, phthalocyanine, triarylmethane pigments etc. (compare Roempp Chemical Lexicon, 9th Edition, Vo. 3,1990 pages 1414/1425.
From what has been stated above it becomes evident that the invention-specific vamish-coated substrate is not transparent. The varnish-coated substrate preferably presents a permeability to light in the visible range of less than approximately 80%, specifically of less than approximately 50%. In a particularly preferred specific embodiment, the varnish-coated substrate is characterized by light permeability of less than approximately 10%; specifically it is light-impermeable.
Preferred within the scope of the invention are specifically acrylic varnishes based on acrylic resins, (compare Roempp Chemical Lexicon, 9th Edition, Volume 1, 1989, page 41). They are applied in many ways in form of solutions in organic solvents, as watery dispersions or as powdery varnishes. To that end, glycolether is a particular suitable and preferred solvent. The following description of the invention is based on the use of a varnish in form of an acrylic resin varnish or an acrylic varnish. As is readily apparent to the person skilled in the art, the determinations made here, correspondingly apply as well with respect to the other previously identified varnish systems if the invention-specific concept is kept in mind of coordination between the varnish and the etching or swelling component of the ink-jet ink. If a colored or black, non-porous surface of a substrate is to be printed for purposes of identification, decoration or similar, then it is of particular benefit to use an acrylic varnish. This acrylic varnish is applied as solution in an organic solvent, as watery dispersion or a powdery varnish and subsequently dried or, after melting, permitted to again cool down. Application is effected in particular by spraying or coating until the non-porous surface to be coated is fully covered. In each instance followed by drying, which causes, on the one hand, the removal of the solvent or dispersing agent, and/or the drying at high temperature after melting of the powdery varnish with subsequent cool-down.
For purposes of the present invention, the thickness of the respectively applied varnish layer is not essential. In general, the varnish layer on the non-porous surface should have a thickness of approximately 1 to approximately 500 xcexcm, specifically approximately 5 to 100 xcexcm.
For purposes of the invention, an inkjet ink has proven itself as particularly suitable which starts to etch or swell specifically with use of an acrylic varnish, which produces good fixation of coloring substance on the substrate surface and which prevents flowing of the inkjet ink.
The varnish layer withdraws from the ink a part of the solvent and thus reduces the drying time. The thereby attained resolution is clearly higher that with printing of the free [uncoated] substrate surface. The type of substrate does not play a significant role. In the end, the only decisive criterion is that the varnish has adequate adhesion on the substrate, which is generally the case. This involves, in particular, substrates of glass, metal, ceramics, plastics and similar. Preferred among the synthetic substances are polystyrol (PS), polyvinyl -chloride (PVC), acryl-nitril-butadiene-styrol copolymerisate (ABS) and polypropylene. Of particular benefit is the invention-specific concept if a non-porous surface of a substrate is printed according to the conventional three color print. With three-color print via inkjet technology, extremely small ink droplets of the basic colors yellow, cyan and magenta are placed next to each other on the substrate, so that, combined with the white background, the impression is created of a mixed color. One speaks here of subtractive color mixing. High quality prints are only possible if bleeding of the ink droplets into each other is prevented, particularly at adjacent color surfaces. The smaller the droplets and the distance between same, the greater the resolution of the print. The invention-specific method provides the required white background and prevents the intermixing of neighboring ink droplets.
The benefits attainable with the invention can be recorded as follows: high resolution after printing, extremely good resistance to smudging and scratching, specifically due to application of white varnish on colored or black substrate for development of color imprints according to the traditional 3-color print. These benefits largely outweigh the disadvantage that an additional production step is needed in realizing the present invention, namely applying the varnish on the non-porous surface of the substrate.